Total energy plants with gas engines show a higher emission of noxious components than other electricity generators. In order to drastically reduce these emissions (NO.sub.x and unbrined components), a three-way catalyst can be used. It is necessary for the proper functioning of the three-way catalyst that the gas-air ratio of the mixture supplied to the gas engine is kept constant. The gas-air ratio can be indicated by the air factor .lambda..
The object of a .lambda. control is to keep the composition of the gas-air mixture supplied to the engine very close to the optimum working point of the three-way catalyst by making use of a closed control circuit. The maximum deviation from the working point is determined by the so-called .lambda. window. Within this window are all the values of .lambda., for which it applies that the emission of all the noxious exhaust gas components remains below the maximum limits fixed for these components.
The lower and upper limits of the .lambda. window are determined by an exhaust gas component which at a lower or higher .lambda. value exceeds the maximum emission permissible for this component. In case of a properly functioning three-way catalyst, the emission of carbon monoxide (CO) is decisive of the lower limit, and the emission of nitrogen oxides (NO.sub.x) is decisive of the upper limit of the .lambda. window. This is shown in FIG. 1.
Earlier research and experiences from demonstration projects show that problems arise with the existing techniques to keep within the .lambda. window. By ageing and wear of the sensor, the air factor gradually changes. The three-way catalyst then no longer works at its optimum working point, and the emissions of noxious substances impermissibly increase. The applicant has done research to solve this problem. This research has led to the development of an optimization method for periodically determining the value of the .lambda. signal which corresponds to the optimum value of the fuel-air ratio for the proper functioning of the three-way catalyst.
In the Netherlands, in contrast to other countries such as Germany, Austria, and Switzerland, few gas engines with a three-way catalyst are used. Much more use is made of another NO.sub.x limiting technique, namely the lean mixture gas engine. Although the potency of these lean mixture techniques is high, the use of particularly catalytic cleaning methods seems unavoidable, if the emission requirements are made more stringent in the future. The branch in which uses with three-way catalysts can be expected soonest is the greenhouse horticulture.
In this branch not only the generated electricity and heat, but also the combustion gases of a total energy plant can be used for CO.sub.2 fertilization, provided these gases do not contain too many noxious components. Apart from saving energy, this is also of economic advantage. For CO.sub.2 fertilization the NO.sub.x emission must be considerably lower then the present legal requirement of 140 g/Gj. Besides, limiting values are also imposed on other noxious components. A three-way catalyst is satisfactory here, as has been demonstrated both in applicant's laboratory and in practice, but exact control of air factor .lambda. by means of a .lambda. sensor is then necessary.
A .lambda. sensor comprises a small sheet of a ceramic material consisting of zirconium dioxide (ZrO.sub.2) stabilized by means of yttrium oxide (Y.sub.2 O.sub.3), provided on both sides with thin platinum electrodes permeable to gas. One of these electrodes comes into contact with the exhaust gases. This electrode functions as a small catalyst. The other electrode is in contact with the ambient air and serves as reference electrode with respect to the oxygen concentration.
It should be observed that only at temperatures above ca. 300.degree. C. is the electrical resistance of the ceramic material sufficiently low for practical use. At this temperature the time lapsed between the moment when changes occur in the gas-air mixture and the moment of change of the .lambda. signal is still in the order of seconds, however. This reaction time of the .lambda. sensor largely depends on the temperature. At a temperature of 600.degree. C. this is reduced to less than 50 ms.
A three-way catalyst converts hydrocarbons, carbon monoxide, and nitrogen oxides having a high to very high conversion efficiency into substances that are not or less noxious. Here it is necessary that the gas-air mixture burned in the engine has an air factor which is only slightly different from 1. In practice, therefore, engines of which the exhaust gases are cleaned by a three-way catalyst are referred to as .lambda.=1 engines. This practically stoichiometric gas-air mixture must be maintained as the optimum working point under all the operating conditions. This stringent requirement cannot even be satisfied by the most advanced fuel systems without a feedback control. It is therefore necessary to use a so-called .lambda. control.
In practice, the .lambda. control operates as follows. Depending on the composition of the exhaust gases, the sensor produces a signal, and depending thereon, the fuel-air ratio is corrected.
The .lambda. sensor is mounted at a location in the exhaust gas system where all the exhaust gases pass.
The width of the .lambda. window is determined by the emission of noxious exhaust gas components by the gas engine before the three-way catalyst (more emission.fwdarw.narrower .lambda. window), the conversion efficiency of the three-way catalyst for each of the noxious components separately, largely depending on the degree of ageing (more ageing.fwdarw.narrower .lambda. window), and the limits of the maximum emission permissible for each of the components (more stringent requirements.fwdarw.narrower .lambda. window). When the .lambda. window becomes narrower, it is necessary to control (even) more accurately. When the .lambda. window has become very narrow, it is better for computer control to have a bit too little gain and too large a time constant. Thus overshoot is prevented and the emission keeps within the limits.
Optimization of the .lambda. control is necessary, because both the .lambda. sensor and the three-way catalyst is susceptible to ageing. Owing to ageing of the three-way catalyst, the .lambda. window changes, and owing to ageing of the .lambda. sensor, the .lambda. signal no longer corresponds to the desired .lambda. value within the .lambda. window. Optimization is necessary to ensure low emissions of noxious components for a longer period of time.
Ageing of the .lambda. sensor and the three-way catalyst depends on the specific use, the process quantities of the use, the number of service hours, the type of catalyst, the size of the catalyst, the type of sensor, the oil consumption of the engine, etc. The ageing process will therefore be different in each situation.
The ageing of the .lambda. sensor manifests itself in general as the gradual decrease of the sensor voltage with an increasing number of service hours at a constant gas-air ratio and constant engine conditions. In the past the applicant already researched this ageing behavior of .lambda. sensors. In general, when used in a gas engine, a .lambda. sensor can stand a long time of use (&gt;10,000 h). In order to prevent thermal damage to the catalytically active outer layer of the ceramic material, the temperature of the sensor may not rise too much. For a longer period of time the maximum temperature of the .lambda. sensor may not exceed ca. 800.degree. C. Higher temperatures lead to damage to the catalytic outer surface of the ceramic material and thus to accelerated ageing.
Another cause of ageing is contamination of the catalytic surface at the outside of the ceramic material.
The .lambda. signal gradually decreases by ageing. The .lambda. control is designed so as to keep the measured .lambda. signal equal to the desired adjusted .lambda. signal. Consequently, owing to the ageing of the .lambda. sensor the gas-air mixture will be adjusted increasingly richer. This is diagrammatically shown in FIG. 2. With the lapse of time the real .lambda. is no longer within the .lambda. window of the three-way catalyst. Thus the emission of specific components of the exhaust gas becomes too high. This ageing is clearly shown in FIG. 3 for a practical situation.
It is clearly visible that at a fixed .lambda. the measured .lambda. signal after, for instance, 5501 service hours, is lower by about 50 mV than after 619 service hours. This difference is practically independent of the .lambda. value. The characteristic of a .lambda. sensor gradually shifts parallel downwards in the graph with the lapse of time. This voltage drop due to ageing of the .lambda. sensor is different for each situation and each sensor. In case of specific .lambda. sensors, immediately after being put into use, the phenomenon may occur that the .lambda. signal increases during the first service hours, before the effect of gradual ageing occurs. In this situation, too, optimization is necessary.
FIG. 2 shows that at a .lambda. sensor signal of 400 mV the fuel-air ratio practically does not change despite ageing of the sensor. In petrol cars equipped with a .lambda. control and a three-way catalyst use is made of this reference point. This reference point is very regularly sought, and then the correcting element is so controlled (on a time base) that a richer mixture is formed. For gas engines this reference point is too remote from the .lambda. sensor signal, which results in low emissions. Very regularly seeking the reference point results in (too) high NO.sub.x emissions, as a result of which this control system cannot be used in stationary gas engines. This control system is known from U.S. Pat. No. 4,526,001.
As in the case of the above ageing of the .lambda. sensor, the service life of the three-way catalyst also largely depends on the exhaust gas temperature prevailing in the catalyst (thermal ageing process) as well as on the type and the concentration of fuel additions. Ageing of a catalyst means that the total active surface area is reduced. Consequently, the conversion efficiency decreases for all the components. This leads to an increasing emission of the noxious exhaust gas components as compared to the situation for a new catalyst. Hence the .lambda. window becomes narrower, since the emission of noxious exhaust gas components increases on ageing, while the requirements for the maximum limits of course remain unchanged. A narrower .lambda. window requires a more accurate adjustment of .lambda.. FIG. 5 shows the effect of the ageing of the three-way catalyst on the emission of the different exhaust gas components. It is clearly visible that the emission levels of C.sub.x H.sub.y, CO, and NO.sub.x rise, and that, moreover, the .lambda. value at which the conversion is optimal shifts to the richer side.
The above clearly shows that there is a need for a system for more or less continuously readjusting or optimizing the .lambda. signal, in relation to the factual emission.